1. Field of the Invention
The present invention relates generally to catalytic converters and, more particularly, to a method for monitoring efficiency of a catalytic converter in a motor vehicle.
2. Description of the Related Art
Increasingly stringent federal and state motor vehicle emission standards include On-Board Diagnostics regulations requiring that specific emission related systems on a motor vehicle be monitored. The intent of the regulations is to ensure that these systems are functioning as intended, and if the system has deteriorated, that the vehicle operator be informed. For example, a catalytic converter of a motor vehicle is monitored because of its ability to reduce undesirable emissions present in exhaust gases from the engine of the motor vehicle.
The catalytic converter may deteriorate over time due to factors such as engine misfire, a faulty oxygen sensor, poisoning, or prolonged high temperature operation. As the catalytic converter deteriorates, it loses its capacity to store the oxygen available in the exhaust gases. This results in an oxygen sensor located downstream of the catalytic converter responding with faster, more frequent switching between a rich and lean condition.
An example of a catalyst monitoring system is described in a Society of Automotive Engineers paper, Number 900062, by Clemmens, Sabourin and Rao. This monitoring system uses a ratio of voltage threshold crossings of oxygen sensors upstream and downstream of the catalytic converter while the engine is operating in an open loop condition. As the catalytic converter deteriorates, and the downstream oxygen sensor begins to switch more frequently, the ratio of voltage threshold crossings approaches one.
One disadvantage of the above catalyst monitoring system is that it imposes an open loop square wave forcing function on the fuel air ratio, making it difficult to maintain the fuel air ratio near stoichiometric under actual driving conditions. Another disadvantage of the catalyst monitoring system is that the resolution is poor if applied to a closed loop condition, and the catalyst monitoring system may indicate that a catalytic converter is deteriorated, when in fact it is good. Consequently, there is a need in the art for monitoring a catalytic converter that does not have a significant effect on maintaining the fuel/air ratio at desired levels. There is also a need in the art for monitoring a catalytic converter that accurately measures the deterioration of the catalytic converter.